// Copyright (c) 2014 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
// extract from chromium source code by @liuwayong
(function () {
	"use strict";
	/**
	 * T-Rex runner.
	 * @param {string} outerContainerId Outer containing element id.
	 * @param {Object} opt_config
	 * @constructor
	 * @export
	 */
	function Runner(outerContainerId, opt_config) {
		// Singleton
		if (Runner.instance_) {
			return Runner.instance_;
		}
		Runner.instance_ = this;

		this.outerContainerEl = document.querySelector(outerContainerId);
		this.containerEl = null;
		this.snackbarEl = null;
		this.detailsButton = this.outerContainerEl.querySelector("#details-button");

		this.config = opt_config || Runner.config;

		this.dimensions = Runner.defaultDimensions;

		this.canvas = null;
		this.canvasCtx = null;

		this.tRex = null;

		this.distanceMeter = null;
		this.distanceRan = 0;

		this.highestScore = 0;

		this.time = 0;
		this.runningTime = 0;
		this.msPerFrame = 1000 / FPS;
		this.currentSpeed = this.config.SPEED;

		this.obstacles = [];

		this.activated = false; // Whether the easter egg has been activated.
		this.playing = false; // Whether the game is currently in play state.
		this.crashed = false;
		this.paused = false;
		this.inverted = false;
		this.invertTimer = 0;
		this.resizeTimerId_ = null;

		this.playCount = 0;

		// Sound FX.
		this.audioBuffer = null;
		this.soundFx = {};

		// Global web audio context for playing sounds.
		this.audioContext = null;

		// Images.
		this.images = {};
		this.imagesLoaded = 0;

		if (this.isDisabled()) {
			this.setupDisabledRunner();
		} else {
			this.loadImages();
		}
	}
	window["Runner"] = Runner;

	/**
	 * Default game width.
	 * @const
	 */
	var DEFAULT_WIDTH = 600;

	/**
	 * Frames per second.
	 * @const
	 */
	var FPS = 60;

	/** @const */
	var IS_HIDPI = window.devicePixelRatio > 1;

	/** @const */
	var IS_IOS = /iPad|iPhone|iPod/.test(window.navigator.platform);

	/** @const */
	var IS_MOBILE = /Android/.test(window.navigator.userAgent) || IS_IOS;

	/** @const */
	var IS_TOUCH_ENABLED = "ontouchstart" in window;

	/**
	 * Default game configuration.
	 * @enum {number}
	 */
	Runner.config = {
		ACCELERATION: 0.001,
		BG_CLOUD_SPEED: 0.2,
		BOTTOM_PAD: 10,
		CLEAR_TIME: 3000,
		CLOUD_FREQUENCY: 0.5,
		GAMEOVER_CLEAR_TIME: 750,
		GAP_COEFFICIENT: 0.6,
		GRAVITY: 0.6,
		INITIAL_JUMP_VELOCITY: 12,
		INVERT_FADE_DURATION: 12000,
		INVERT_DISTANCE: 700,
		MAX_BLINK_COUNT: 3,
		MAX_CLOUDS: 6,
		MAX_OBSTACLE_LENGTH: 3,
		MAX_OBSTACLE_DUPLICATION: 2,
		MAX_SPEED: 13,
		MIN_JUMP_HEIGHT: 35,
		MOBILE_SPEED_COEFFICIENT: 1.2,
		RESOURCE_TEMPLATE_ID: "audio-resources",
		SPEED: 6,
		SPEED_DROP_COEFFICIENT: 3,
		ARCADE_MODE_INITIAL_TOP_POSITION: 35,
		ARCADE_MODE_TOP_POSITION_PERCENT: 0.1
	};

	/**
	 * Default dimensions.
	 * @enum {string}
	 */
	Runner.defaultDimensions = {
		WIDTH: DEFAULT_WIDTH,
		HEIGHT: 150
	};

	/**
	 * CSS class names.
	 * @enum {string}
	 */
	Runner.classes = {
		ARCADE_MODE: "arcade-mode",
		CANVAS: "runner-canvas",
		CONTAINER: "runner-container",
		CRASHED: "crashed",
		ICON: "icon-offline",
		INVERTED: "inverted",
		SNACKBAR: "snackbar",
		SNACKBAR_SHOW: "snackbar-show",
		TOUCH_CONTROLLER: "controller"
	};

	/**
	 * Sprite definition layout of the spritesheet.
	 * @enum {Object}
	 */
	Runner.spriteDefinition = {
		LDPI: {
			CACTUS_LARGE: { x: 332, y: 2 },
			CACTUS_SMALL: { x: 228, y: 2 },
			CLOUD: { x: 86, y: 2 },
			HORIZON: { x: 2, y: 54 },
			MOON: { x: 484, y: 2 },
			PTERODACTYL: { x: 134, y: 2 },
			RESTART: { x: 2, y: 2 },
			TEXT_SPRITE: { x: 655, y: 2 },
			TREX: { x: 848, y: 2 },
			STAR: { x: 645, y: 2 }
		},
		HDPI: {
			CACTUS_LARGE: { x: 652, y: 2 },
			CACTUS_SMALL: { x: 446, y: 2 },
			CLOUD: { x: 166, y: 2 },
			HORIZON: { x: 2, y: 104 },
			MOON: { x: 954, y: 2 },
			PTERODACTYL: { x: 260, y: 2 },
			RESTART: { x: 2, y: 2 },
			TEXT_SPRITE: { x: 1294, y: 2 },
			TREX: { x: 1678, y: 2 },
			STAR: { x: 1276, y: 2 }
		}
	};

	/**
	 * Sound FX. Reference to the ID of the audio tag on interstitial page.
	 * @enum {string}
	 */
	Runner.sounds = {
		BUTTON_PRESS: "offline-sound-press",
		HIT: "offline-sound-hit",
		SCORE: "offline-sound-reached"
	};

	/**
	 * Key code mapping.
	 * @enum {Object}
	 */
	Runner.keycodes = {
		JUMP: { 38: 1, 32: 1 }, // Up, spacebar
		DUCK: { 40: 1 }, // Down
		RESTART: { 13: 1 } // Enter
	};

	/**
	 * Runner event names.
	 * @enum {string}
	 */
	Runner.events = {
		ANIM_END: "webkitAnimationEnd",
		CLICK: "click",
		KEYDOWN: "keydown",
		KEYUP: "keyup",
		MOUSEDOWN: "mousedown",
		MOUSEUP: "mouseup",
		RESIZE: "resize",
		TOUCHEND: "touchend",
		TOUCHSTART: "touchstart",
		VISIBILITY: "visibilitychange",
		BLUR: "blur",
		FOCUS: "focus",
		LOAD: "load"
	};

	Runner.prototype = {
		/**
		 * Whether the easter egg has been disabled. CrOS enterprise enrolled devices.
		 * @return {boolean}
		 */
		isDisabled: function () {
			// return loadTimeData && loadTimeData.valueExists('disabledEasterEgg');
			return false;
		},

		/**
		 * For disabled instances, set up a snackbar with the disabled message.
		 */
		setupDisabledRunner: function () {
			this.containerEl = document.createElement("div");
			this.containerEl.className = Runner.classes.SNACKBAR;
			this.containerEl.textContent = loadTimeData.getValue("disabledEasterEgg");
			this.outerContainerEl.appendChild(this.containerEl);

			// Show notification when the activation key is pressed.
			document.addEventListener(
				Runner.events.KEYDOWN,
				function (e) {
					if (Runner.keycodes.JUMP[e.keyCode]) {
						this.containerEl.classList.add(Runner.classes.SNACKBAR_SHOW);
						document.querySelector(".icon").classList.add("icon-disabled");
					}
				}.bind(this)
			);
		},

		/**
		 * Setting individual settings for debugging.
		 * @param {string} setting
		 * @param {*} value
		 */
		updateConfigSetting: function (setting, value) {
			if (setting in this.config && value != undefined) {
				this.config[setting] = value;

				switch (setting) {
					case "GRAVITY":
					case "MIN_JUMP_HEIGHT":
					case "SPEED_DROP_COEFFICIENT":
						this.tRex.config[setting] = value;
						break;
					case "INITIAL_JUMP_VELOCITY":
						this.tRex.setJumpVelocity(value);
						break;
					case "SPEED":
						this.setSpeed(value);
						break;
				}
			}
		},

		/**
		 * Cache the appropriate image sprite from the page and get the sprite sheet
		 * definition.
		 */
		loadImages: function () {
			if (IS_HIDPI) {
				Runner.imageSprite = document.getElementById("offline-resources-2x");
				this.spriteDef = Runner.spriteDefinition.HDPI;
			} else {
				Runner.imageSprite = document.getElementById("offline-resources-1x");
				this.spriteDef = Runner.spriteDefinition.LDPI;
			}

			if (Runner.imageSprite.complete) {
				this.init();
			} else {
				// If the images are not yet loaded, add a listener.
				Runner.imageSprite.addEventListener(Runner.events.LOAD, this.init.bind(this));
			}
		},

		/**
		 * Load and decode base 64 encoded sounds.
		 */
		loadSounds: function () {
			if (!IS_IOS) {
				this.audioContext = new AudioContext();

				var resourceTemplate = document.getElementById(this.config.RESOURCE_TEMPLATE_ID).content;

				for (var sound in Runner.sounds) {
					var soundSrc = resourceTemplate.getElementById(Runner.sounds[sound]).src;
					soundSrc = soundSrc.substr(soundSrc.indexOf(",") + 1);
					var buffer = decodeBase64ToArrayBuffer(soundSrc);

					// Async, so no guarantee of order in array.
					this.audioContext.decodeAudioData(
						buffer,
						function (index, audioData) {
							this.soundFx[index] = audioData;
						}.bind(this, sound)
					);
				}
			}
		},

		/**
		 * Sets the game speed. Adjust the speed accordingly if on a smaller screen.
		 * @param {number} opt_speed
		 */
		setSpeed: function (opt_speed) {
			var speed = opt_speed || this.currentSpeed;

			// Reduce the speed on smaller mobile screens.
			if (this.dimensions.WIDTH < DEFAULT_WIDTH) {
				var mobileSpeed = ((speed * this.dimensions.WIDTH) / DEFAULT_WIDTH) * this.config.MOBILE_SPEED_COEFFICIENT;
				this.currentSpeed = mobileSpeed > speed ? speed : mobileSpeed;
			} else if (opt_speed) {
				this.currentSpeed = opt_speed;
			}
		},

		/**
		 * Game initialiser.
		 */
		init: function () {
			// Hide the static icon.
			document.querySelector("." + Runner.classes.ICON).style.visibility = "hidden";

			this.adjustDimensions();
			this.setSpeed();

			this.containerEl = document.createElement("div");
			this.containerEl.className = Runner.classes.CONTAINER;

			// Player canvas container.
			this.canvas = createCanvas(
				this.containerEl,
				this.dimensions.WIDTH,
				this.dimensions.HEIGHT,
				Runner.classes.PLAYER
			);

			this.canvasCtx = this.canvas.getContext("2d");
			this.canvasCtx.fillStyle = "#f7f7f7";
			this.canvasCtx.fill();
			Runner.updateCanvasScaling(this.canvas);

			// Horizon contains clouds, obstacles and the ground.
			this.horizon = new Horizon(this.canvas, this.spriteDef, this.dimensions, this.config.GAP_COEFFICIENT);

			// Distance meter
			this.distanceMeter = new DistanceMeter(this.canvas, this.spriteDef.TEXT_SPRITE, this.dimensions.WIDTH);

			// Draw t-rex
			this.tRex = new Trex(this.canvas, this.spriteDef.TREX);

			this.outerContainerEl.appendChild(this.containerEl);

			if (IS_MOBILE) {
				this.createTouchController();
			}

			this.startListening();
			this.update();

			window.addEventListener(Runner.events.RESIZE, this.debounceResize.bind(this));
		},

		/**
		 * Create the touch controller. A div that covers whole screen.
		 */
		createTouchController: function () {
			this.touchController = document.createElement("div");
			this.touchController.className = Runner.classes.TOUCH_CONTROLLER;
			this.outerContainerEl.appendChild(this.touchController);
		},

		/**
		 * Debounce the resize event.
		 */
		debounceResize: function () {
			if (!this.resizeTimerId_) {
				this.resizeTimerId_ = setInterval(this.adjustDimensions.bind(this), 250);
			}
		},

		/**
		 * Adjust game space dimensions on resize.
		 */
		adjustDimensions: function () {
			clearInterval(this.resizeTimerId_);
			this.resizeTimerId_ = null;

			var boxStyles = window.getComputedStyle(this.outerContainerEl);
			var padding = Number(boxStyles.paddingLeft.substr(0, boxStyles.paddingLeft.length - 2));

			this.dimensions.WIDTH = this.outerContainerEl.offsetWidth - padding * 2;
			this.dimensions.WIDTH = Math.min(DEFAULT_WIDTH, this.dimensions.WIDTH); //Arcade Mode
			if (this.activated) {
				this.setArcadeModeContainerScale();
			}

			// Redraw the elements back onto the canvas.
			if (this.canvas) {
				this.canvas.width = this.dimensions.WIDTH;
				this.canvas.height = this.dimensions.HEIGHT;

				Runner.updateCanvasScaling(this.canvas);

				this.distanceMeter.calcXPos(this.dimensions.WIDTH);
				this.clearCanvas();
				this.horizon.update(0, 0, true);
				this.tRex.update(0);

				// Outer container and distance meter.
				if (this.playing || this.crashed || this.paused) {
					this.containerEl.style.width = this.dimensions.WIDTH + "px";
					this.containerEl.style.height = this.dimensions.HEIGHT + "px";
					this.distanceMeter.update(0, Math.ceil(this.distanceRan));
					this.stop();
				} else {
					this.tRex.draw(0, 0);
				}

				// Game over panel.
				if (this.crashed && this.gameOverPanel) {
					this.gameOverPanel.updateDimensions(this.dimensions.WIDTH);
					this.gameOverPanel.draw();
				}
			}
		},

		/**
		 * Play the game intro.
		 * Canvas container width expands out to the full width.
		 */
		playIntro: function () {
			if (!this.activated && !this.crashed) {
				this.playingIntro = true;
				this.tRex.playingIntro = true;

				// CSS animation definition.
				var keyframes =
					"@-webkit-keyframes intro { " +
					"from { width:" +
					Trex.config.WIDTH +
					"px }" +
					"to { width: " +
					this.dimensions.WIDTH +
					"px }" +
					"}";

				// create a style sheet to put the keyframe rule in
				// and then place the style sheet in the html head
				var sheet = document.createElement("style");
				sheet.innerHTML = keyframes;
				document.head.appendChild(sheet);

				this.containerEl.addEventListener(Runner.events.ANIM_END, this.startGame.bind(this));

				this.containerEl.style.webkitAnimation = "intro .4s ease-out 1 both";
				this.containerEl.style.width = this.dimensions.WIDTH + "px";

				// if (this.touchController) {
				//     this.outerContainerEl.appendChild(this.touchController);
				// }
				this.playing = true;
				this.activated = true;
			} else if (this.crashed) {
				this.restart();
			}
		},

		/**
		 * Update the game status to started.
		 */
		startGame: function () {
			this.setArcadeMode();
			this.runningTime = 0;
			this.playingIntro = false;
			this.tRex.playingIntro = false;
			this.containerEl.style.webkitAnimation = "";
			this.playCount++;

			// Handle tabbing off the page. Pause the current game.
			document.addEventListener(Runner.events.VISIBILITY, this.onVisibilityChange.bind(this));

			window.addEventListener(Runner.events.BLUR, this.onVisibilityChange.bind(this));

			window.addEventListener(Runner.events.FOCUS, this.onVisibilityChange.bind(this));
		},

		clearCanvas: function () {
			this.canvasCtx.clearRect(0, 0, this.dimensions.WIDTH, this.dimensions.HEIGHT);
		},

		/**
		 * Update the game frame and schedules the next one.
		 */
		update: function () {
			this.updatePending = false;

			var now = getTimeStamp();
			var deltaTime = now - (this.time || now);
			this.time = now;

			if (this.playing) {
				this.clearCanvas();

				if (this.tRex.jumping) {
					this.tRex.updateJump(deltaTime);
				}

				this.runningTime += deltaTime;
				var hasObstacles = this.runningTime > this.config.CLEAR_TIME;

				// First jump triggers the intro.
				if (this.tRex.jumpCount == 1 && !this.playingIntro) {
					this.playIntro();
				}

				// The horizon doesn't move until the intro is over.
				if (this.playingIntro) {
					this.horizon.update(0, this.currentSpeed, hasObstacles);
				} else {
					deltaTime = !this.activated ? 0 : deltaTime;
					this.horizon.update(deltaTime, this.currentSpeed, hasObstacles, this.inverted);
				}

				// Check for collisions.
				var collision = hasObstacles && checkForCollision(this.horizon.obstacles[0], this.tRex);

				if (!collision) {
					this.distanceRan += (this.currentSpeed * deltaTime) / this.msPerFrame;

					if (this.currentSpeed < this.config.MAX_SPEED) {
						this.currentSpeed += this.config.ACCELERATION;
					}
				} else {
					this.gameOver();
				}

				var playAchievementSound = this.distanceMeter.update(deltaTime, Math.ceil(this.distanceRan));

				if (playAchievementSound) {
					this.playSound(this.soundFx.SCORE);
				}

				// Night mode.
				if (this.invertTimer > this.config.INVERT_FADE_DURATION) {
					this.invertTimer = 0;
					this.invertTrigger = false;
					this.invert();
				} else if (this.invertTimer) {
					this.invertTimer += deltaTime;
				} else {
					var actualDistance = this.distanceMeter.getActualDistance(Math.ceil(this.distanceRan));

					if (actualDistance > 0) {
						this.invertTrigger = !(actualDistance % this.config.INVERT_DISTANCE);

						if (this.invertTrigger && this.invertTimer === 0) {
							this.invertTimer += deltaTime;
							this.invert();
						}
					}
				}
			}

			if (this.playing || (!this.activated && this.tRex.blinkCount < Runner.config.MAX_BLINK_COUNT)) {
				this.tRex.update(deltaTime);
				this.scheduleNextUpdate();
			}
		},

		/**
		 * Event handler.
		 */
		handleEvent: function (e) {
			return function (evtType, events) {
				switch (evtType) {
					case events.KEYDOWN:
					case events.TOUCHSTART:
					case events.MOUSEDOWN:
						this.onKeyDown(e);
						break;
					case events.KEYUP:
					case events.TOUCHEND:
					case events.MOUSEUP:
						this.onKeyUp(e);
						break;
				}
			}.bind(this)(e.type, Runner.events);
		},

		/**
		 * Bind relevant key / mouse / touch listeners.
		 */
		startListening: function () {
			// Keys.
			document.addEventListener(Runner.events.KEYDOWN, this);
			document.addEventListener(Runner.events.KEYUP, this);

			if (IS_MOBILE) {
				// Mobile only touch devices.
				this.touchController.addEventListener(Runner.events.TOUCHSTART, this);
				this.touchController.addEventListener(Runner.events.TOUCHEND, this);
				this.containerEl.addEventListener(Runner.events.TOUCHSTART, this);
			} else {
				// Mouse.
				document.addEventListener(Runner.events.MOUSEDOWN, this);
				document.addEventListener(Runner.events.MOUSEUP, this);
			}
		},

		/**
		 * Remove all listeners.
		 */
		stopListening: function () {
			document.removeEventListener(Runner.events.KEYDOWN, this);
			document.removeEventListener(Runner.events.KEYUP, this);

			if (IS_MOBILE) {
				this.touchController.removeEventListener(Runner.events.TOUCHSTART, this);
				this.touchController.removeEventListener(Runner.events.TOUCHEND, this);
				this.containerEl.removeEventListener(Runner.events.TOUCHSTART, this);
			} else {
				document.removeEventListener(Runner.events.MOUSEDOWN, this);
				document.removeEventListener(Runner.events.MOUSEUP, this);
			}
		},

		/**
		 * Process keydown.
		 * @param {Event} e
		 */
		onKeyDown: function (e) {
			// Prevent native page scrolling whilst tapping on mobile.
			if (IS_MOBILE && this.playing) {
				e.preventDefault();
			}

			if (e.target != this.detailsButton) {
				if (!this.crashed && (Runner.keycodes.JUMP[e.keyCode] || e.type == Runner.events.TOUCHSTART)) {
					if (!this.playing) {
						this.loadSounds();
						this.playing = true;
						this.update();
						if (window.errorPageController) {
							errorPageController.trackEasterEgg();
						}
					}
					//  Play sound effect and jump on starting the game for the first time.
					if (!this.tRex.jumping && !this.tRex.ducking) {
						this.playSound(this.soundFx.BUTTON_PRESS);
						this.tRex.startJump(this.currentSpeed);
					}
				}

				if (this.crashed && e.type == Runner.events.TOUCHSTART && e.currentTarget == this.containerEl) {
					this.restart();
				}
			}

			if (this.playing && !this.crashed && Runner.keycodes.DUCK[e.keyCode]) {
				e.preventDefault();
				if (this.tRex.jumping) {
					// Speed drop, activated only when jump key is not pressed.
					this.tRex.setSpeedDrop();
				} else if (!this.tRex.jumping && !this.tRex.ducking) {
					// Duck.
					this.tRex.setDuck(true);
				}
			}
		},

		/**
		 * Process key up.
		 * @param {Event} e
		 */
		onKeyUp: function (e) {
			var keyCode = String(e.keyCode);
			var isjumpKey =
				Runner.keycodes.JUMP[keyCode] || e.type == Runner.events.TOUCHEND || e.type == Runner.events.MOUSEDOWN;

			if (this.isRunning() && isjumpKey) {
				this.tRex.endJump();
			} else if (Runner.keycodes.DUCK[keyCode]) {
				this.tRex.speedDrop = false;
				this.tRex.setDuck(false);
			} else if (this.crashed) {
				// Check that enough time has elapsed before allowing jump key to restart.
				var deltaTime = getTimeStamp() - this.time;

				if (
					Runner.keycodes.RESTART[keyCode] ||
					this.isLeftClickOnCanvas(e) ||
					(deltaTime >= this.config.GAMEOVER_CLEAR_TIME && Runner.keycodes.JUMP[keyCode])
				) {
					this.restart();
				}
			} else if (this.paused && isjumpKey) {
				// Reset the jump state
				this.tRex.reset();
				this.play();
			}
		},

		/**
		 * Returns whether the event was a left click on canvas.
		 * On Windows right click is registered as a click.
		 * @param {Event} e
		 * @return {boolean}
		 */
		isLeftClickOnCanvas: function (e) {
			return e.button != null && e.button < 2 && e.type == Runner.events.MOUSEUP && e.target == this.canvas;
		},

		/**
		 * RequestAnimationFrame wrapper.
		 */
		scheduleNextUpdate: function () {
			if (!this.updatePending) {
				this.updatePending = true;
				this.raqId = requestAnimationFrame(this.update.bind(this));
			}
		},

		/**
		 * Whether the game is running.
		 * @return {boolean}
		 */
		isRunning: function () {
			return !!this.raqId;
		},

		/**
		 * Game over state.
		 */
		gameOver: function () {
			this.playSound(this.soundFx.HIT);
			vibrate(200);

			this.stop();
			this.crashed = true;
			this.distanceMeter.acheivement = false;

			this.tRex.update(100, Trex.status.CRASHED);

			// Game over panel.
			if (!this.gameOverPanel) {
				this.gameOverPanel = new GameOverPanel(
					this.canvas,
					this.spriteDef.TEXT_SPRITE,
					this.spriteDef.RESTART,
					this.dimensions
				);
			} else {
				this.gameOverPanel.draw();
			}

			// Update the high score.
			if (this.distanceRan > this.highestScore) {
				this.highestScore = Math.ceil(this.distanceRan);
				this.distanceMeter.setHighScore(this.highestScore);
			}

			// Reset the time clock.
			this.time = getTimeStamp();
		},

		stop: function () {
			this.playing = false;
			this.paused = true;
			cancelAnimationFrame(this.raqId);
			this.raqId = 0;
		},

		play: function () {
			if (!this.crashed) {
				this.playing = true;
				this.paused = false;
				this.tRex.update(0, Trex.status.RUNNING);
				this.time = getTimeStamp();
				this.update();
			}
		},

		restart: function () {
			if (!this.raqId) {
				this.playCount++;
				this.runningTime = 0;
				this.playing = true;
				this.crashed = false;
				this.distanceRan = 0;
				this.setSpeed(this.config.SPEED);
				this.time = getTimeStamp();
				this.containerEl.classList.remove(Runner.classes.CRASHED);
				this.clearCanvas();
				this.distanceMeter.reset(this.highestScore);
				this.horizon.reset();
				this.tRex.reset();
				this.playSound(this.soundFx.BUTTON_PRESS);
				this.invert(true);
				this.update();
			}
		},

		/**
		 * Hides offline messaging for a fullscreen game only experience.
		 */
		setArcadeMode() {
			document.body.classList.add(Runner.classes.ARCADE_MODE);
			this.setArcadeModeContainerScale();
		},

		/**
		 * Sets the scaling for arcade mode.
		 */
		setArcadeModeContainerScale() {
			const windowHeight = window.innerHeight;
			const scaleHeight = windowHeight / this.dimensions.HEIGHT;
			const scaleWidth = window.innerWidth / this.dimensions.WIDTH;
			const scale = Math.max(1, Math.min(scaleHeight, scaleWidth));
			const scaledCanvasHeight = this.dimensions.HEIGHT * scale;
			// Positions the game container at 10% of the available vertical window
			// height minus the game container height.
			const translateY =
				Math.ceil(
					Math.max(
						0,
						(windowHeight - scaledCanvasHeight - Runner.config.ARCADE_MODE_INITIAL_TOP_POSITION) *
							Runner.config.ARCADE_MODE_TOP_POSITION_PERCENT
					)
				) * window.devicePixelRatio;

			const cssScale = scale;
			this.containerEl.style.transform = "scale(" + cssScale + ") translateY(" + translateY + "px)";
		},

		/**
		 * Pause the game if the tab is not in focus.
		 */
		onVisibilityChange: function (e) {
			if (document.hidden || document.webkitHidden || e.type == "blur" || document.visibilityState != "visible") {
				this.stop();
			} else if (!this.crashed) {
				this.tRex.reset();
				this.play();
			}
		},

		/**
		 * Play a sound.
		 * @param {SoundBuffer} soundBuffer
		 */
		playSound: function (soundBuffer) {
			if (soundBuffer) {
				var sourceNode = this.audioContext.createBufferSource();
				sourceNode.buffer = soundBuffer;
				sourceNode.connect(this.audioContext.destination);
				sourceNode.start(0);
			}
		},

		/**
		 * Inverts the current page / canvas colors.
		 * @param {boolean} Whether to reset colors.
		 */
		invert: function (reset) {
			if (reset) {
				document.body.classList.toggle(Runner.classes.INVERTED, false);
				this.invertTimer = 0;
				this.inverted = false;
			} else {
				this.inverted = document.body.classList.toggle(Runner.classes.INVERTED, this.invertTrigger);
			}
		}
	};

	/**
	 * Updates the canvas size taking into
	 * account the backing store pixel ratio and
	 * the device pixel ratio.
	 *
	 * See article by Paul Lewis:
	 * http://www.html5rocks.com/en/tutorials/canvas/hidpi/
	 *
	 * @param {HTMLCanvasElement} canvas
	 * @param {number} opt_width
	 * @param {number} opt_height
	 * @return {boolean} Whether the canvas was scaled.
	 */
	Runner.updateCanvasScaling = function (canvas, opt_width, opt_height) {
		var context = canvas.getContext("2d");

		// Query the various pixel ratios
		var devicePixelRatio = Math.floor(window.devicePixelRatio) || 1;
		var backingStoreRatio = Math.floor(context.webkitBackingStorePixelRatio) || 1;
		var ratio = devicePixelRatio / backingStoreRatio;

		// Upscale the canvas if the two ratios don't match
		if (devicePixelRatio !== backingStoreRatio) {
			var oldWidth = opt_width || canvas.width;
			var oldHeight = opt_height || canvas.height;

			canvas.width = oldWidth * ratio;
			canvas.height = oldHeight * ratio;

			canvas.style.width = oldWidth + "px";
			canvas.style.height = oldHeight + "px";

			// Scale the context to counter the fact that we've manually scaled
			// our canvas element.
			context.scale(ratio, ratio);
			return true;
		} else if (devicePixelRatio == 1) {
			// Reset the canvas width / height. Fixes scaling bug when the page is
			// zoomed and the devicePixelRatio changes accordingly.
			canvas.style.width = canvas.width + "px";
			canvas.style.height = canvas.height + "px";
		}
		return false;
	};

	/**
	 * Get random number.
	 * @param {number} min
	 * @param {number} max
	 * @param {number}
	 */
	function getRandomNum(min, max) {
		return Math.floor(Math.random() * (max - min + 1)) + min;
	}

	/**
	 * Vibrate on mobile devices.
	 * @param {number} duration Duration of the vibration in milliseconds.
	 */
	function vibrate(duration) {
		if (IS_MOBILE && window.navigator.vibrate) {
			window.navigator.vibrate(duration);
		}
	}

	/**
	 * Create canvas element.
	 * @param {HTMLElement} container Element to append canvas to.
	 * @param {number} width
	 * @param {number} height
	 * @param {string} opt_classname
	 * @return {HTMLCanvasElement}
	 */
	function createCanvas(container, width, height, opt_classname) {
		var canvas = document.createElement("canvas");
		canvas.className = opt_classname ? Runner.classes.CANVAS + " " + opt_classname : Runner.classes.CANVAS;
		canvas.width = width;
		canvas.height = height;
		container.appendChild(canvas);

		return canvas;
	}

	/**
	 * Decodes the base 64 audio to ArrayBuffer used by Web Audio.
	 * @param {string} base64String
	 */
	function decodeBase64ToArrayBuffer(base64String) {
		var len = (base64String.length / 4) * 3;
		var str = atob(base64String);
		var arrayBuffer = new ArrayBuffer(len);
		var bytes = new Uint8Array(arrayBuffer);

		for (var i = 0; i < len; i++) {
			bytes[i] = str.charCodeAt(i);
		}
		return bytes.buffer;
	}

	/**
	 * Return the current timestamp.
	 * @return {number}
	 */
	function getTimeStamp() {
		return IS_IOS ? new Date().getTime() : performance.now();
	}

	//******************************************************************************

	/**
	 * Game over panel.
	 * @param {!HTMLCanvasElement} canvas
	 * @param {Object} textImgPos
	 * @param {Object} restartImgPos
	 * @param {!Object} dimensions Canvas dimensions.
	 * @constructor
	 */
	function GameOverPanel(canvas, textImgPos, restartImgPos, dimensions) {
		this.canvas = canvas;
		this.canvasCtx = canvas.getContext("2d");
		this.canvasDimensions = dimensions;
		this.textImgPos = textImgPos;
		this.restartImgPos = restartImgPos;
		this.draw();
	}

	/**
	 * Dimensions used in the panel.
	 * @enum {number}
	 */
	GameOverPanel.dimensions = {
		TEXT_X: 0,
		TEXT_Y: 13,
		TEXT_WIDTH: 191,
		TEXT_HEIGHT: 11,
		RESTART_WIDTH: 36,
		RESTART_HEIGHT: 32
	};

	GameOverPanel.prototype = {
		/**
		 * Update the panel dimensions.
		 * @param {number} width New canvas width.
		 * @param {number} opt_height Optional new canvas height.
		 */
		updateDimensions: function (width, opt_height) {
			this.canvasDimensions.WIDTH = width;
			if (opt_height) {
				this.canvasDimensions.HEIGHT = opt_height;
			}
		},

		/**
		 * Draw the panel.
		 */
		draw: function () {
			var dimensions = GameOverPanel.dimensions;

			var centerX = this.canvasDimensions.WIDTH / 2;

			// Game over text.
			var textSourceX = dimensions.TEXT_X;
			var textSourceY = dimensions.TEXT_Y;
			var textSourceWidth = dimensions.TEXT_WIDTH;
			var textSourceHeight = dimensions.TEXT_HEIGHT;

			var textTargetX = Math.round(centerX - dimensions.TEXT_WIDTH / 2);
			var textTargetY = Math.round((this.canvasDimensions.HEIGHT - 25) / 3);
			var textTargetWidth = dimensions.TEXT_WIDTH;
			var textTargetHeight = dimensions.TEXT_HEIGHT;

			var restartSourceWidth = dimensions.RESTART_WIDTH;
			var restartSourceHeight = dimensions.RESTART_HEIGHT;
			var restartTargetX = centerX - dimensions.RESTART_WIDTH / 2;
			var restartTargetY = this.canvasDimensions.HEIGHT / 2;

			if (IS_HIDPI) {
				textSourceY *= 2;
				textSourceX *= 2;
				textSourceWidth *= 2;
				textSourceHeight *= 2;
				restartSourceWidth *= 2;
				restartSourceHeight *= 2;
			}

			textSourceX += this.textImgPos.x;
			textSourceY += this.textImgPos.y;

			// Game over text from sprite.
			this.canvasCtx.drawImage(
				Runner.imageSprite,
				textSourceX,
				textSourceY,
				textSourceWidth,
				textSourceHeight,
				textTargetX,
				textTargetY,
				textTargetWidth,
				textTargetHeight
			);

			// Restart button.
			this.canvasCtx.drawImage(
				Runner.imageSprite,
				this.restartImgPos.x,
				this.restartImgPos.y,
				restartSourceWidth,
				restartSourceHeight,
				restartTargetX,
				restartTargetY,
				dimensions.RESTART_WIDTH,
				dimensions.RESTART_HEIGHT
			);
		}
	};

	//******************************************************************************

	/**
	 * Check for a collision.
	 * @param {!Obstacle} obstacle
	 * @param {!Trex} tRex T-rex object.
	 * @param {HTMLCanvasContext} opt_canvasCtx Optional canvas context for drawing
	 *    collision boxes.
	 * @return {Array<CollisionBox>}
	 */
	function checkForCollision(obstacle, tRex, opt_canvasCtx) {
		var obstacleBoxXPos = Runner.defaultDimensions.WIDTH + obstacle.xPos;

		// Adjustments are made to the bounding box as there is a 1 pixel white
		// border around the t-rex and obstacles.
		var tRexBox = new CollisionBox(tRex.xPos + 1, tRex.yPos + 1, tRex.config.WIDTH - 2, tRex.config.HEIGHT - 2);

		var obstacleBox = new CollisionBox(
			obstacle.xPos + 1,
			obstacle.yPos + 1,
			obstacle.typeConfig.width * obstacle.size - 2,
			obstacle.typeConfig.height - 2
		);

		// Debug outer box
		if (opt_canvasCtx) {
			drawCollisionBoxes(opt_canvasCtx, tRexBox, obstacleBox);
		}

		// Simple outer bounds check.
		if (boxCompare(tRexBox, obstacleBox)) {
			var collisionBoxes = obstacle.collisionBoxes;
			var tRexCollisionBoxes = tRex.ducking ? Trex.collisionBoxes.DUCKING : Trex.collisionBoxes.RUNNING;

			// Detailed axis aligned box check.
			for (var t = 0; t < tRexCollisionBoxes.length; t++) {
				for (var i = 0; i < collisionBoxes.length; i++) {
					// Adjust the box to actual positions.
					var adjTrexBox = createAdjustedCollisionBox(tRexCollisionBoxes[t], tRexBox);
					var adjObstacleBox = createAdjustedCollisionBox(collisionBoxes[i], obstacleBox);
					var crashed = boxCompare(adjTrexBox, adjObstacleBox);

					// Draw boxes for debug.
					if (opt_canvasCtx) {
						drawCollisionBoxes(opt_canvasCtx, adjTrexBox, adjObstacleBox);
					}

					if (crashed) {
						return [adjTrexBox, adjObstacleBox];
					}
				}
			}
		}
		return false;
	}

	/**
	 * Adjust the collision box.
	 * @param {!CollisionBox} box The original box.
	 * @param {!CollisionBox} adjustment Adjustment box.
	 * @return {CollisionBox} The adjusted collision box object.
	 */
	function createAdjustedCollisionBox(box, adjustment) {
		return new CollisionBox(box.x + adjustment.x, box.y + adjustment.y, box.width, box.height);
	}

	/**
	 * Draw the collision boxes for debug.
	 */
	function drawCollisionBoxes(canvasCtx, tRexBox, obstacleBox) {
		canvasCtx.save();
		canvasCtx.strokeStyle = "#f00";
		canvasCtx.strokeRect(tRexBox.x, tRexBox.y, tRexBox.width, tRexBox.height);

		canvasCtx.strokeStyle = "#0f0";
		canvasCtx.strokeRect(obstacleBox.x, obstacleBox.y, obstacleBox.width, obstacleBox.height);
		canvasCtx.restore();
	}

	/**
	 * Compare two collision boxes for a collision.
	 * @param {CollisionBox} tRexBox
	 * @param {CollisionBox} obstacleBox
	 * @return {boolean} Whether the boxes intersected.
	 */
	function boxCompare(tRexBox, obstacleBox) {
		var crashed = false;
		var tRexBoxX = tRexBox.x;
		var tRexBoxY = tRexBox.y;

		var obstacleBoxX = obstacleBox.x;
		var obstacleBoxY = obstacleBox.y;

		// Axis-Aligned Bounding Box method.
		if (
			tRexBox.x < obstacleBoxX + obstacleBox.width &&
			tRexBox.x + tRexBox.width > obstacleBoxX &&
			tRexBox.y < obstacleBox.y + obstacleBox.height &&
			tRexBox.height + tRexBox.y > obstacleBox.y
		) {
			crashed = true;
		}

		return crashed;
	}

	//******************************************************************************

	/**
	 * Collision box object.
	 * @param {number} x X position.
	 * @param {number} y Y Position.
	 * @param {number} w Width.
	 * @param {number} h Height.
	 */
	function CollisionBox(x, y, w, h) {
		this.x = x;
		this.y = y;
		this.width = w;
		this.height = h;
	}

	//******************************************************************************

	/**
	 * Obstacle.
	 * @param {HTMLCanvasCtx} canvasCtx
	 * @param {Obstacle.type} type
	 * @param {Object} spritePos Obstacle position in sprite.
	 * @param {Object} dimensions
	 * @param {number} gapCoefficient Mutipler in determining the gap.
	 * @param {number} speed
	 * @param {number} opt_xOffset
	 */
	function Obstacle(canvasCtx, type, spriteImgPos, dimensions, gapCoefficient, speed, opt_xOffset) {
		this.canvasCtx = canvasCtx;
		this.spritePos = spriteImgPos;
		this.typeConfig = type;
		this.gapCoefficient = gapCoefficient;
		this.size = getRandomNum(1, Obstacle.MAX_OBSTACLE_LENGTH);
		this.dimensions = dimensions;
		this.remove = false;
		this.xPos = dimensions.WIDTH + (opt_xOffset || 0);
		this.yPos = 0;
		this.width = 0;
		this.collisionBoxes = [];
		this.gap = 0;
		this.speedOffset = 0;

		// For animated obstacles.
		this.currentFrame = 0;
		this.timer = 0;

		this.init(speed);
	}

	/**
	 * Coefficient for calculating the maximum gap.
	 * @const
	 */
	Obstacle.MAX_GAP_COEFFICIENT = 1.5;

	/**
	 * Maximum obstacle grouping count.
	 * @const
	 */
	(Obstacle.MAX_OBSTACLE_LENGTH = 3),
		(Obstacle.prototype = {
			/**
			 * Initialise the DOM for the obstacle.
			 * @param {number} speed
			 */
			init: function (speed) {
				this.cloneCollisionBoxes();

				// Only allow sizing if we're at the right speed.
				if (this.size > 1 && this.typeConfig.multipleSpeed > speed) {
					this.size = 1;
				}

				this.width = this.typeConfig.width * this.size;

				// Check if obstacle can be positioned at various heights.
				if (Array.isArray(this.typeConfig.yPos)) {
					var yPosConfig = IS_MOBILE ? this.typeConfig.yPosMobile : this.typeConfig.yPos;
					this.yPos = yPosConfig[getRandomNum(0, yPosConfig.length - 1)];
				} else {
					this.yPos = this.typeConfig.yPos;
				}

				this.draw();

				// Make collision box adjustments,
				// Central box is adjusted to the size as one box.
				//      ____        ______        ________
				//    _|   |-|    _|     |-|    _|       |-|
				//   | |<->| |   | |<--->| |   | |<----->| |
				//   | | 1 | |   | |  2  | |   | |   3   | |
				//   |_|___|_|   |_|_____|_|   |_|_______|_|
				//
				if (this.size > 1) {
					this.collisionBoxes[1].width = this.width - this.collisionBoxes[0].width - this.collisionBoxes[2].width;
					this.collisionBoxes[2].x = this.width - this.collisionBoxes[2].width;
				}

				// For obstacles that go at a different speed from the horizon.
				if (this.typeConfig.speedOffset) {
					this.speedOffset = Math.random() > 0.5 ? this.typeConfig.speedOffset : -this.typeConfig.speedOffset;
				}

				this.gap = this.getGap(this.gapCoefficient, speed);
			},

			/**
			 * Draw and crop based on size.
			 */
			draw: function () {
				var sourceWidth = this.typeConfig.width;
				var sourceHeight = this.typeConfig.height;

				if (IS_HIDPI) {
					sourceWidth = sourceWidth * 2;
					sourceHeight = sourceHeight * 2;
				}

				// X position in sprite.
				var sourceX = sourceWidth * this.size * (0.5 * (this.size - 1)) + this.spritePos.x;

				// Animation frames.
				if (this.currentFrame > 0) {
					sourceX += sourceWidth * this.currentFrame;
				}

				this.canvasCtx.drawImage(
					Runner.imageSprite,
					sourceX,
					this.spritePos.y,
					sourceWidth * this.size,
					sourceHeight,
					this.xPos,
					this.yPos,
					this.typeConfig.width * this.size,
					this.typeConfig.height
				);
			},

			/**
			 * Obstacle frame update.
			 * @param {number} deltaTime
			 * @param {number} speed
			 */
			update: function (deltaTime, speed) {
				if (!this.remove) {
					if (this.typeConfig.speedOffset) {
						speed += this.speedOffset;
					}
					this.xPos -= Math.floor(((speed * FPS) / 1000) * deltaTime);

					// Update frame
					if (this.typeConfig.numFrames) {
						this.timer += deltaTime;
						if (this.timer >= this.typeConfig.frameRate) {
							this.currentFrame = this.currentFrame == this.typeConfig.numFrames - 1 ? 0 : this.currentFrame + 1;
							this.timer = 0;
						}
					}
					this.draw();

					if (!this.isVisible()) {
						this.remove = true;
					}
				}
			},

			/**
			 * Calculate a random gap size.
			 * - Minimum gap gets wider as speed increses
			 * @param {number} gapCoefficient
			 * @param {number} speed
			 * @return {number} The gap size.
			 */
			getGap: function (gapCoefficient, speed) {
				var minGap = Math.round(this.width * speed + this.typeConfig.minGap * gapCoefficient);
				var maxGap = Math.round(minGap * Obstacle.MAX_GAP_COEFFICIENT);
				return getRandomNum(minGap, maxGap);
			},

			/**
			 * Check if obstacle is visible.
			 * @return {boolean} Whether the obstacle is in the game area.
			 */
			isVisible: function () {
				return this.xPos + this.width > 0;
			},

			/**
			 * Make a copy of the collision boxes, since these will change based on
			 * obstacle type and size.
			 */
			cloneCollisionBoxes: function () {
				var collisionBoxes = this.typeConfig.collisionBoxes;

				for (var i = collisionBoxes.length - 1; i >= 0; i--) {
					this.collisionBoxes[i] = new CollisionBox(
						collisionBoxes[i].x,
						collisionBoxes[i].y,
						collisionBoxes[i].width,
						collisionBoxes[i].height
					);
				}
			}
		});

	/**
	 * Obstacle definitions.
	 * minGap: minimum pixel space betweeen obstacles.
	 * multipleSpeed: Speed at which multiples are allowed.
	 * speedOffset: speed faster / slower than the horizon.
	 * minSpeed: Minimum speed which the obstacle can make an appearance.
	 */
	Obstacle.types = [
		{
			type: "CACTUS_SMALL",
			width: 17,
			height: 35,
			yPos: 105,
			multipleSpeed: 4,
			minGap: 120,
			minSpeed: 0,
			collisionBoxes: [new CollisionBox(0, 7, 5, 27), new CollisionBox(4, 0, 6, 34), new CollisionBox(10, 4, 7, 14)]
		},
		{
			type: "CACTUS_LARGE",
			width: 25,
			height: 50,
			yPos: 90,
			multipleSpeed: 7,
			minGap: 120,
			minSpeed: 0,
			collisionBoxes: [new CollisionBox(0, 12, 7, 38), new CollisionBox(8, 0, 7, 49), new CollisionBox(13, 10, 10, 38)]
		},
		{
			type: "PTERODACTYL",
			width: 46,
			height: 40,
			yPos: [100, 75, 50], // Variable height.
			yPosMobile: [100, 50], // Variable height mobile.
			multipleSpeed: 999,
			minSpeed: 8.5,
			minGap: 150,
			collisionBoxes: [
				new CollisionBox(15, 15, 16, 5),
				new CollisionBox(18, 21, 24, 6),
				new CollisionBox(2, 14, 4, 3),
				new CollisionBox(6, 10, 4, 7),
				new CollisionBox(10, 8, 6, 9)
			],
			numFrames: 2,
			frameRate: 1000 / 6,
			speedOffset: 0.8
		}
	];

	//******************************************************************************
	/**
	 * T-rex game character.
	 * @param {HTMLCanvas} canvas
	 * @param {Object} spritePos Positioning within image sprite.
	 * @constructor
	 */
	function Trex(canvas, spritePos) {
		this.canvas = canvas;
		this.canvasCtx = canvas.getContext("2d");
		this.spritePos = spritePos;
		this.xPos = 0;
		this.yPos = 0;
		// Position when on the ground.
		this.groundYPos = 0;
		this.currentFrame = 0;
		this.currentAnimFrames = [];
		this.blinkDelay = 0;
		this.blinkCount = 0;
		this.animStartTime = 0;
		this.timer = 0;
		this.msPerFrame = 1000 / FPS;
		this.config = Trex.config;
		// Current status.
		this.status = Trex.status.WAITING;

		this.jumping = false;
		this.ducking = false;
		this.jumpVelocity = 0;
		this.reachedMinHeight = false;
		this.speedDrop = false;
		this.jumpCount = 0;
		this.jumpspotX = 0;

		this.init();
	}

	/**
	 * T-rex player config.
	 * @enum {number}
	 */
	Trex.config = {
		DROP_VELOCITY: -5,
		GRAVITY: 0.6,
		HEIGHT: 47,
		HEIGHT_DUCK: 25,
		INIITAL_JUMP_VELOCITY: -10,
		INTRO_DURATION: 1500,
		MAX_JUMP_HEIGHT: 30,
		MIN_JUMP_HEIGHT: 30,
		SPEED_DROP_COEFFICIENT: 3,
		SPRITE_WIDTH: 262,
		START_X_POS: 50,
		WIDTH: 44,
		WIDTH_DUCK: 59
	};

	/**
	 * Used in collision detection.
	 * @type {Array<CollisionBox>}
	 */
	Trex.collisionBoxes = {
		DUCKING: [new CollisionBox(1, 18, 55, 25)],
		RUNNING: [
			new CollisionBox(22, 0, 17, 16),
			new CollisionBox(1, 18, 30, 9),
			new CollisionBox(10, 35, 14, 8),
			new CollisionBox(1, 24, 29, 5),
			new CollisionBox(5, 30, 21, 4),
			new CollisionBox(9, 34, 15, 4)
		]
	};

	/**
	 * Animation states.
	 * @enum {string}
	 */
	Trex.status = {
		CRASHED: "CRASHED",
		DUCKING: "DUCKING",
		JUMPING: "JUMPING",
		RUNNING: "RUNNING",
		WAITING: "WAITING"
	};

	/**
	 * Blinking coefficient.
	 * @const
	 */
	Trex.BLINK_TIMING = 7000;

	/**
	 * Animation config for different states.
	 * @enum {Object}
	 */
	Trex.animFrames = {
		WAITING: {
			frames: [44, 0],
			msPerFrame: 1000 / 3
		},
		RUNNING: {
			frames: [88, 132],
			msPerFrame: 1000 / 12
		},
		CRASHED: {
			frames: [220],
			msPerFrame: 1000 / 60
		},
		JUMPING: {
			frames: [0],
			msPerFrame: 1000 / 60
		},
		DUCKING: {
			frames: [264, 323],
			msPerFrame: 1000 / 8
		}
	};

	Trex.prototype = {
		/**
		 * T-rex player initaliser.
		 * Sets the t-rex to blink at random intervals.
		 */
		init: function () {
			this.groundYPos = Runner.defaultDimensions.HEIGHT - this.config.HEIGHT - Runner.config.BOTTOM_PAD;
			this.yPos = this.groundYPos;
			this.minJumpHeight = this.groundYPos - this.config.MIN_JUMP_HEIGHT;

			this.draw(0, 0);
			this.update(0, Trex.status.WAITING);
		},

		/**
		 * Setter for the jump velocity.
		 * The approriate drop velocity is also set.
		 */
		setJumpVelocity: function (setting) {
			this.config.INIITAL_JUMP_VELOCITY = -setting;
			this.config.DROP_VELOCITY = -setting / 2;
		},

		/**
		 * Set the animation status.
		 * @param {!number} deltaTime
		 * @param {Trex.status} status Optional status to switch to.
		 */
		update: function (deltaTime, opt_status) {
			this.timer += deltaTime;

			// Update the status.
			if (opt_status) {
				this.status = opt_status;
				this.currentFrame = 0;
				this.msPerFrame = Trex.animFrames[opt_status].msPerFrame;
				this.currentAnimFrames = Trex.animFrames[opt_status].frames;

				if (opt_status == Trex.status.WAITING) {
					this.animStartTime = getTimeStamp();
					this.setBlinkDelay();
				}
			}

			// Game intro animation, T-rex moves in from the left.
			if (this.playingIntro && this.xPos < this.config.START_X_POS) {
				this.xPos += Math.round((this.config.START_X_POS / this.config.INTRO_DURATION) * deltaTime);
			}

			if (this.status == Trex.status.WAITING) {
				this.blink(getTimeStamp());
			} else {
				this.draw(this.currentAnimFrames[this.currentFrame], 0);
			}

			// Update the frame position.
			if (this.timer >= this.msPerFrame) {
				this.currentFrame = this.currentFrame == this.currentAnimFrames.length - 1 ? 0 : this.currentFrame + 1;
				this.timer = 0;
			}

			// Speed drop becomes duck if the down key is still being pressed.
			if (this.speedDrop && this.yPos == this.groundYPos) {
				this.speedDrop = false;
				this.setDuck(true);
			}
		},

		/**
		 * Draw the t-rex to a particular position.
		 * @param {number} x
		 * @param {number} y
		 */
		draw: function (x, y) {
			var sourceX = x;
			var sourceY = y;
			var sourceWidth = this.ducking && this.status != Trex.status.CRASHED ? this.config.WIDTH_DUCK : this.config.WIDTH;
			var sourceHeight = this.config.HEIGHT;

			if (IS_HIDPI) {
				sourceX *= 2;
				sourceY *= 2;
				sourceWidth *= 2;
				sourceHeight *= 2;
			}

			// Adjustments for sprite sheet position.
			sourceX += this.spritePos.x;
			sourceY += this.spritePos.y;

			// Ducking.
			if (this.ducking && this.status != Trex.status.CRASHED) {
				this.canvasCtx.drawImage(
					Runner.imageSprite,
					sourceX,
					sourceY,
					sourceWidth,
					sourceHeight,
					this.xPos,
					this.yPos,
					this.config.WIDTH_DUCK,
					this.config.HEIGHT
				);
			} else {
				// Crashed whilst ducking. Trex is standing up so needs adjustment.
				if (this.ducking && this.status == Trex.status.CRASHED) {
					this.xPos++;
				}
				// Standing / running
				this.canvasCtx.drawImage(
					Runner.imageSprite,
					sourceX,
					sourceY,
					sourceWidth,
					sourceHeight,
					this.xPos,
					this.yPos,
					this.config.WIDTH,
					this.config.HEIGHT
				);
			}
		},

		/**
		 * Sets a random time for the blink to happen.
		 */
		setBlinkDelay: function () {
			this.blinkDelay = Math.ceil(Math.random() * Trex.BLINK_TIMING);
		},

		/**
		 * Make t-rex blink at random intervals.
		 * @param {number} time Current time in milliseconds.
		 */
		blink: function (time) {
			var deltaTime = time - this.animStartTime;

			if (deltaTime >= this.blinkDelay) {
				this.draw(this.currentAnimFrames[this.currentFrame], 0);

				if (this.currentFrame == 1) {
					// Set new random delay to blink.
					this.setBlinkDelay();
					this.animStartTime = time;
					this.blinkCount++;
				}
			}
		},

		/**
		 * Initialise a jump.
		 * @param {number} speed
		 */
		startJump: function (speed) {
			if (!this.jumping) {
				this.update(0, Trex.status.JUMPING);
				// Tweak the jump velocity based on the speed.
				this.jumpVelocity = this.config.INIITAL_JUMP_VELOCITY - speed / 10;
				this.jumping = true;
				this.reachedMinHeight = false;
				this.speedDrop = false;
			}
		},

		/**
		 * Jump is complete, falling down.
		 */
		endJump: function () {
			if (this.reachedMinHeight && this.jumpVelocity < this.config.DROP_VELOCITY) {
				this.jumpVelocity = this.config.DROP_VELOCITY;
			}
		},

		/**
		 * Update frame for a jump.
		 * @param {number} deltaTime
		 * @param {number} speed
		 */
		updateJump: function (deltaTime, speed) {
			var msPerFrame = Trex.animFrames[this.status].msPerFrame;
			var framesElapsed = deltaTime / msPerFrame;

			// Speed drop makes Trex fall faster.
			if (this.speedDrop) {
				this.yPos += Math.round(this.jumpVelocity * this.config.SPEED_DROP_COEFFICIENT * framesElapsed);
			} else {
				this.yPos += Math.round(this.jumpVelocity * framesElapsed);
			}

			this.jumpVelocity += this.config.GRAVITY * framesElapsed;

			// Minimum height has been reached.
			if (this.yPos < this.minJumpHeight || this.speedDrop) {
				this.reachedMinHeight = true;
			}

			// Reached max height
			if (this.yPos < this.config.MAX_JUMP_HEIGHT || this.speedDrop) {
				this.endJump();
			}

			// Back down at ground level. Jump completed.
			if (this.yPos > this.groundYPos) {
				this.reset();
				this.jumpCount++;
			}

			this.update(deltaTime);
		},

		/**
		 * Set the speed drop. Immediately cancels the current jump.
		 */
		setSpeedDrop: function () {
			this.speedDrop = true;
			this.jumpVelocity = 1;
		},

		/**
		 * @param {boolean} isDucking.
		 */
		setDuck: function (isDucking) {
			if (isDucking && this.status != Trex.status.DUCKING) {
				this.update(0, Trex.status.DUCKING);
				this.ducking = true;
			} else if (this.status == Trex.status.DUCKING) {
				this.update(0, Trex.status.RUNNING);
				this.ducking = false;
			}
		},

		/**
		 * Reset the t-rex to running at start of game.
		 */
		reset: function () {
			this.yPos = this.groundYPos;
			this.jumpVelocity = 0;
			this.jumping = false;
			this.ducking = false;
			this.update(0, Trex.status.RUNNING);
			this.midair = false;
			this.speedDrop = false;
			this.jumpCount = 0;
		}
	};

	//******************************************************************************

	/**
	 * Handles displaying the distance meter.
	 * @param {!HTMLCanvasElement} canvas
	 * @param {Object} spritePos Image position in sprite.
	 * @param {number} canvasWidth
	 * @constructor
	 */
	function DistanceMeter(canvas, spritePos, canvasWidth) {
		this.canvas = canvas;
		this.canvasCtx = canvas.getContext("2d");
		this.image = Runner.imageSprite;
		this.spritePos = spritePos;
		this.x = 0;
		this.y = 5;

		this.currentDistance = 0;
		this.maxScore = 0;
		this.highScore = 0;
		this.container = null;

		this.digits = [];
		this.acheivement = false;
		this.defaultString = "";
		this.flashTimer = 0;
		this.flashIterations = 0;
		this.invertTrigger = false;

		this.config = DistanceMeter.config;
		this.maxScoreUnits = this.config.MAX_DISTANCE_UNITS;
		this.init(canvasWidth);
	}

	/**
	 * @enum {number}
	 */
	DistanceMeter.dimensions = {
		WIDTH: 10,
		HEIGHT: 13,
		DEST_WIDTH: 11
	};

	/**
	 * Y positioning of the digits in the sprite sheet.
	 * X position is always 0.
	 * @type {Array<number>}
	 */
	DistanceMeter.yPos = [0, 13, 27, 40, 53, 67, 80, 93, 107, 120];

	/**
	 * Distance meter config.
	 * @enum {number}
	 */
	DistanceMeter.config = {
		// Number of digits.
		MAX_DISTANCE_UNITS: 5,

		// Distance that causes achievement animation.
		ACHIEVEMENT_DISTANCE: 100,

		// Used for conversion from pixel distance to a scaled unit.
		COEFFICIENT: 0.025,

		// Flash duration in milliseconds.
		FLASH_DURATION: 1000 / 4,

		// Flash iterations for achievement animation.
		FLASH_ITERATIONS: 3
	};

	DistanceMeter.prototype = {
		/**
		 * Initialise the distance meter to '00000'.
		 * @param {number} width Canvas width in px.
		 */
		init: function (width) {
			var maxDistanceStr = "";

			this.calcXPos(width);
			this.maxScore = this.maxScoreUnits;
			for (var i = 0; i < this.maxScoreUnits; i++) {
				this.draw(i, 0);
				this.defaultString += "0";
				maxDistanceStr += "9";
			}

			this.maxScore = parseInt(maxDistanceStr);
		},

		/**
		 * Calculate the xPos in the canvas.
		 * @param {number} canvasWidth
		 */
		calcXPos: function (canvasWidth) {
			this.x = canvasWidth - DistanceMeter.dimensions.DEST_WIDTH * (this.maxScoreUnits + 1);
		},

		/**
		 * Draw a digit to canvas.
		 * @param {number} digitPos Position of the digit.
		 * @param {number} value Digit value 0-9.
		 * @param {boolean} opt_highScore Whether drawing the high score.
		 */
		draw: function (digitPos, value, opt_highScore) {
			var sourceWidth = DistanceMeter.dimensions.WIDTH;
			var sourceHeight = DistanceMeter.dimensions.HEIGHT;
			var sourceX = DistanceMeter.dimensions.WIDTH * value;
			var sourceY = 0;

			var targetX = digitPos * DistanceMeter.dimensions.DEST_WIDTH;
			var targetY = this.y;
			var targetWidth = DistanceMeter.dimensions.WIDTH;
			var targetHeight = DistanceMeter.dimensions.HEIGHT;

			// For high DPI we 2x source values.
			if (IS_HIDPI) {
				sourceWidth *= 2;
				sourceHeight *= 2;
				sourceX *= 2;
			}

			sourceX += this.spritePos.x;
			sourceY += this.spritePos.y;

			this.canvasCtx.save();

			if (opt_highScore) {
				// Left of the current score.
				var highScoreX = this.x - this.maxScoreUnits * 2 * DistanceMeter.dimensions.WIDTH;
				this.canvasCtx.translate(highScoreX, this.y);
			} else {
				this.canvasCtx.translate(this.x, this.y);
			}

			this.canvasCtx.drawImage(
				this.image,
				sourceX,
				sourceY,
				sourceWidth,
				sourceHeight,
				targetX,
				targetY,
				targetWidth,
				targetHeight
			);

			this.canvasCtx.restore();
		},

		/**
		 * Covert pixel distance to a 'real' distance.
		 * @param {number} distance Pixel distance ran.
		 * @return {number} The 'real' distance ran.
		 */
		getActualDistance: function (distance) {
			return distance ? Math.round(distance * this.config.COEFFICIENT) : 0;
		},

		/**
		 * Update the distance meter.
		 * @param {number} distance
		 * @param {number} deltaTime
		 * @return {boolean} Whether the acheivement sound fx should be played.
		 */
		update: function (deltaTime, distance) {
			var paint = true;
			var playSound = false;

			if (!this.acheivement) {
				distance = this.getActualDistance(distance);
				// Score has gone beyond the initial digit count.
				if (distance > this.maxScore && this.maxScoreUnits == this.config.MAX_DISTANCE_UNITS) {
					this.maxScoreUnits++;
					this.maxScore = parseInt(this.maxScore + "9");
				} else {
					this.distance = 0;
				}

				if (distance > 0) {
					// Acheivement unlocked
					if (distance % this.config.ACHIEVEMENT_DISTANCE == 0) {
						// Flash score and play sound.
						this.acheivement = true;
						this.flashTimer = 0;
						playSound = true;
					}

					// Create a string representation of the distance with leading 0.
					var distanceStr = (this.defaultString + distance).substr(-this.maxScoreUnits);
					this.digits = distanceStr.split("");
				} else {
					this.digits = this.defaultString.split("");
				}
			} else {
				// Control flashing of the score on reaching acheivement.
				if (this.flashIterations <= this.config.FLASH_ITERATIONS) {
					this.flashTimer += deltaTime;

					if (this.flashTimer < this.config.FLASH_DURATION) {
						paint = false;
					} else if (this.flashTimer > this.config.FLASH_DURATION * 2) {
						this.flashTimer = 0;
						this.flashIterations++;
					}
				} else {
					this.acheivement = false;
					this.flashIterations = 0;
					this.flashTimer = 0;
				}
			}

			// Draw the digits if not flashing.
			if (paint) {
				for (var i = this.digits.length - 1; i >= 0; i--) {
					this.draw(i, parseInt(this.digits[i]));
				}
			}

			this.drawHighScore();
			return playSound;
		},

		/**
		 * Draw the high score.
		 */
		drawHighScore: function () {
			this.canvasCtx.save();
			this.canvasCtx.globalAlpha = 0.8;
			for (var i = this.highScore.length - 1; i >= 0; i--) {
				this.draw(i, parseInt(this.highScore[i], 10), true);
			}
			this.canvasCtx.restore();
		},

		/**
		 * Set the highscore as a array string.
		 * Position of char in the sprite: H - 10, I - 11.
		 * @param {number} distance Distance ran in pixels.
		 */
		setHighScore: function (distance) {
			distance = this.getActualDistance(distance);
			var highScoreStr = (this.defaultString + distance).substr(-this.maxScoreUnits);

			this.highScore = ["10", "11", ""].concat(highScoreStr.split(""));
		},

		/**
		 * Reset the distance meter back to '00000'.
		 */
		reset: function () {
			this.update(0);
			this.acheivement = false;
		}
	};

	//******************************************************************************

	/**
	 * Cloud background item.
	 * Similar to an obstacle object but without collision boxes.
	 * @param {HTMLCanvasElement} canvas Canvas element.
	 * @param {Object} spritePos Position of image in sprite.
	 * @param {number} containerWidth
	 */
	function Cloud(canvas, spritePos, containerWidth) {
		this.canvas = canvas;
		this.canvasCtx = this.canvas.getContext("2d");
		this.spritePos = spritePos;
		this.containerWidth = containerWidth;
		this.xPos = containerWidth;
		this.yPos = 0;
		this.remove = false;
		this.cloudGap = getRandomNum(Cloud.config.MIN_CLOUD_GAP, Cloud.config.MAX_CLOUD_GAP);

		this.init();
	}

	/**
	 * Cloud object config.
	 * @enum {number}
	 */
	Cloud.config = {
		HEIGHT: 14,
		MAX_CLOUD_GAP: 400,
		MAX_SKY_LEVEL: 30,
		MIN_CLOUD_GAP: 100,
		MIN_SKY_LEVEL: 71,
		WIDTH: 46
	};

	Cloud.prototype = {
		/**
		 * Initialise the cloud. Sets the Cloud height.
		 */
		init: function () {
			this.yPos = getRandomNum(Cloud.config.MAX_SKY_LEVEL, Cloud.config.MIN_SKY_LEVEL);
			this.draw();
		},

		/**
		 * Draw the cloud.
		 */
		draw: function () {
			this.canvasCtx.save();
			var sourceWidth = Cloud.config.WIDTH;
			var sourceHeight = Cloud.config.HEIGHT;

			if (IS_HIDPI) {
				sourceWidth = sourceWidth * 2;
				sourceHeight = sourceHeight * 2;
			}

			this.canvasCtx.drawImage(
				Runner.imageSprite,
				this.spritePos.x,
				this.spritePos.y,
				sourceWidth,
				sourceHeight,
				this.xPos,
				this.yPos,
				Cloud.config.WIDTH,
				Cloud.config.HEIGHT
			);

			this.canvasCtx.restore();
		},

		/**
		 * Update the cloud position.
		 * @param {number} speed
		 */
		update: function (speed) {
			if (!this.remove) {
				this.xPos -= Math.ceil(speed);
				this.draw();

				// Mark as removeable if no longer in the canvas.
				if (!this.isVisible()) {
					this.remove = true;
				}
			}
		},

		/**
		 * Check if the cloud is visible on the stage.
		 * @return {boolean}
		 */
		isVisible: function () {
			return this.xPos + Cloud.config.WIDTH > 0;
		}
	};

	//******************************************************************************

	/**
	 * Nightmode shows a moon and stars on the horizon.
	 */
	function NightMode(canvas, spritePos, containerWidth) {
		this.spritePos = spritePos;
		this.canvas = canvas;
		this.canvasCtx = canvas.getContext("2d");
		this.xPos = containerWidth - 50;
		this.yPos = 30;
		this.currentPhase = 0;
		this.opacity = 0;
		this.containerWidth = containerWidth;
		this.stars = [];
		this.drawStars = false;
		this.placeStars();
	}

	/**
	 * @enum {number}
	 */
	NightMode.config = {
		FADE_SPEED: 0.035,
		HEIGHT: 40,
		MOON_SPEED: 0.25,
		NUM_STARS: 2,
		STAR_SIZE: 9,
		STAR_SPEED: 0.3,
		STAR_MAX_Y: 70,
		WIDTH: 20
	};

	NightMode.phases = [140, 120, 100, 60, 40, 20, 0];

	NightMode.prototype = {
		/**
		 * Update moving moon, changing phases.
		 * @param {boolean} activated Whether night mode is activated.
		 * @param {number} delta
		 */
		update: function (activated, delta) {
			// Moon phase.
			if (activated && this.opacity == 0) {
				this.currentPhase++;

				if (this.currentPhase >= NightMode.phases.length) {
					this.currentPhase = 0;
				}
			}

			// Fade in / out.
			if (activated && (this.opacity < 1 || this.opacity == 0)) {
				this.opacity += NightMode.config.FADE_SPEED;
			} else if (this.opacity > 0) {
				this.opacity -= NightMode.config.FADE_SPEED;
			}

			// Set moon positioning.
			if (this.opacity > 0) {
				this.xPos = this.updateXPos(this.xPos, NightMode.config.MOON_SPEED);

				// Update stars.
				if (this.drawStars) {
					for (var i = 0; i < NightMode.config.NUM_STARS; i++) {
						this.stars[i].x = this.updateXPos(this.stars[i].x, NightMode.config.STAR_SPEED);
					}
				}
				this.draw();
			} else {
				this.opacity = 0;
				this.placeStars();
			}
			this.drawStars = true;
		},

		updateXPos: function (currentPos, speed) {
			if (currentPos < -NightMode.config.WIDTH) {
				currentPos = this.containerWidth;
			} else {
				currentPos -= speed;
			}
			return currentPos;
		},

		draw: function () {
			var moonSourceWidth = this.currentPhase == 3 ? NightMode.config.WIDTH * 2 : NightMode.config.WIDTH;
			var moonSourceHeight = NightMode.config.HEIGHT;
			var moonSourceX = this.spritePos.x + NightMode.phases[this.currentPhase];
			var moonOutputWidth = moonSourceWidth;
			var starSize = NightMode.config.STAR_SIZE;
			var starSourceX = Runner.spriteDefinition.LDPI.STAR.x;

			if (IS_HIDPI) {
				moonSourceWidth *= 2;
				moonSourceHeight *= 2;
				moonSourceX = this.spritePos.x + NightMode.phases[this.currentPhase] * 2;
				starSize *= 2;
				starSourceX = Runner.spriteDefinition.HDPI.STAR.x;
			}

			this.canvasCtx.save();
			this.canvasCtx.globalAlpha = this.opacity;

			// Stars.
			if (this.drawStars) {
				for (var i = 0; i < NightMode.config.NUM_STARS; i++) {
					this.canvasCtx.drawImage(
						Runner.imageSprite,
						starSourceX,
						this.stars[i].sourceY,
						starSize,
						starSize,
						Math.round(this.stars[i].x),
						this.stars[i].y,
						NightMode.config.STAR_SIZE,
						NightMode.config.STAR_SIZE
					);
				}
			}

			// Moon.
			this.canvasCtx.drawImage(
				Runner.imageSprite,
				moonSourceX,
				this.spritePos.y,
				moonSourceWidth,
				moonSourceHeight,
				Math.round(this.xPos),
				this.yPos,
				moonOutputWidth,
				NightMode.config.HEIGHT
			);

			this.canvasCtx.globalAlpha = 1;
			this.canvasCtx.restore();
		},

		// Do star placement.
		placeStars: function () {
			var segmentSize = Math.round(this.containerWidth / NightMode.config.NUM_STARS);

			for (var i = 0; i < NightMode.config.NUM_STARS; i++) {
				this.stars[i] = {};
				this.stars[i].x = getRandomNum(segmentSize * i, segmentSize * (i + 1));
				this.stars[i].y = getRandomNum(0, NightMode.config.STAR_MAX_Y);

				if (IS_HIDPI) {
					this.stars[i].sourceY = Runner.spriteDefinition.HDPI.STAR.y + NightMode.config.STAR_SIZE * 2 * i;
				} else {
					this.stars[i].sourceY = Runner.spriteDefinition.LDPI.STAR.y + NightMode.config.STAR_SIZE * i;
				}
			}
		},

		reset: function () {
			this.currentPhase = 0;
			this.opacity = 0;
			this.update(false);
		}
	};

	//******************************************************************************

	/**
	 * Horizon Line.
	 * Consists of two connecting lines. Randomly assigns a flat / bumpy horizon.
	 * @param {HTMLCanvasElement} canvas
	 * @param {Object} spritePos Horizon position in sprite.
	 * @constructor
	 */
	function HorizonLine(canvas, spritePos) {
		this.spritePos = spritePos;
		this.canvas = canvas;
		this.canvasCtx = canvas.getContext("2d");
		this.sourceDimensions = {};
		this.dimensions = HorizonLine.dimensions;
		this.sourceXPos = [this.spritePos.x, this.spritePos.x + this.dimensions.WIDTH];
		this.xPos = [];
		this.yPos = 0;
		this.bumpThreshold = 0.5;

		this.setSourceDimensions();
		this.draw();
	}

	/**
	 * Horizon line dimensions.
	 * @enum {number}
	 */
	HorizonLine.dimensions = {
		WIDTH: 600,
		HEIGHT: 12,
		YPOS: 127
	};

	HorizonLine.prototype = {
		/**
		 * Set the source dimensions of the horizon line.
		 */
		setSourceDimensions: function () {
			for (var dimension in HorizonLine.dimensions) {
				if (IS_HIDPI) {
					if (dimension != "YPOS") {
						this.sourceDimensions[dimension] = HorizonLine.dimensions[dimension] * 2;
					}
				} else {
					this.sourceDimensions[dimension] = HorizonLine.dimensions[dimension];
				}
				this.dimensions[dimension] = HorizonLine.dimensions[dimension];
			}

			this.xPos = [0, HorizonLine.dimensions.WIDTH];
			this.yPos = HorizonLine.dimensions.YPOS;
		},

		/**
		 * Return the crop x position of a type.
		 */
		getRandomType: function () {
			return Math.random() > this.bumpThreshold ? this.dimensions.WIDTH : 0;
		},

		/**
		 * Draw the horizon line.
		 */
		draw: function () {
			this.canvasCtx.drawImage(
				Runner.imageSprite,
				this.sourceXPos[0],
				this.spritePos.y,
				this.sourceDimensions.WIDTH,
				this.sourceDimensions.HEIGHT,
				this.xPos[0],
				this.yPos,
				this.dimensions.WIDTH,
				this.dimensions.HEIGHT
			);

			this.canvasCtx.drawImage(
				Runner.imageSprite,
				this.sourceXPos[1],
				this.spritePos.y,
				this.sourceDimensions.WIDTH,
				this.sourceDimensions.HEIGHT,
				this.xPos[1],
				this.yPos,
				this.dimensions.WIDTH,
				this.dimensions.HEIGHT
			);
		},

		/**
		 * Update the x position of an indivdual piece of the line.
		 * @param {number} pos Line position.
		 * @param {number} increment
		 */
		updateXPos: function (pos, increment) {
			var line1 = pos;
			var line2 = pos == 0 ? 1 : 0;

			this.xPos[line1] -= increment;
			this.xPos[line2] = this.xPos[line1] + this.dimensions.WIDTH;

			if (this.xPos[line1] <= -this.dimensions.WIDTH) {
				this.xPos[line1] += this.dimensions.WIDTH * 2;
				this.xPos[line2] = this.xPos[line1] - this.dimensions.WIDTH;
				this.sourceXPos[line1] = this.getRandomType() + this.spritePos.x;
			}
		},

		/**
		 * Update the horizon line.
		 * @param {number} deltaTime
		 * @param {number} speed
		 */
		update: function (deltaTime, speed) {
			var increment = Math.floor(speed * (FPS / 1000) * deltaTime);

			if (this.xPos[0] <= 0) {
				this.updateXPos(0, increment);
			} else {
				this.updateXPos(1, increment);
			}
			this.draw();
		},

		/**
		 * Reset horizon to the starting position.
		 */
		reset: function () {
			this.xPos[0] = 0;
			this.xPos[1] = HorizonLine.dimensions.WIDTH;
		}
	};

	//******************************************************************************

	/**
	 * Horizon background class.
	 * @param {HTMLCanvasElement} canvas
	 * @param {Object} spritePos Sprite positioning.
	 * @param {Object} dimensions Canvas dimensions.
	 * @param {number} gapCoefficient
	 * @constructor
	 */
	function Horizon(canvas, spritePos, dimensions, gapCoefficient) {
		this.canvas = canvas;
		this.canvasCtx = this.canvas.getContext("2d");
		this.config = Horizon.config;
		this.dimensions = dimensions;
		this.gapCoefficient = gapCoefficient;
		this.obstacles = [];
		this.obstacleHistory = [];
		this.horizonOffsets = [0, 0];
		this.cloudFrequency = this.config.CLOUD_FREQUENCY;
		this.spritePos = spritePos;
		this.nightMode = null;

		// Cloud
		this.clouds = [];
		this.cloudSpeed = this.config.BG_CLOUD_SPEED;

		// Horizon
		this.horizonLine = null;
		this.init();
	}

	/**
	 * Horizon config.
	 * @enum {number}
	 */
	Horizon.config = {
		BG_CLOUD_SPEED: 0.2,
		BUMPY_THRESHOLD: 0.3,
		CLOUD_FREQUENCY: 0.5,
		HORIZON_HEIGHT: 16,
		MAX_CLOUDS: 6
	};

	Horizon.prototype = {
		/**
		 * Initialise the horizon. Just add the line and a cloud. No obstacles.
		 */
		init: function () {
			this.addCloud();
			this.horizonLine = new HorizonLine(this.canvas, this.spritePos.HORIZON);
			this.nightMode = new NightMode(this.canvas, this.spritePos.MOON, this.dimensions.WIDTH);
		},

		/**
		 * @param {number} deltaTime
		 * @param {number} currentSpeed
		 * @param {boolean} updateObstacles Used as an override to prevent
		 *     the obstacles from being updated / added. This happens in the
		 *     ease in section.
		 * @param {boolean} showNightMode Night mode activated.
		 */
		update: function (deltaTime, currentSpeed, updateObstacles, showNightMode) {
			this.runningTime += deltaTime;
			this.horizonLine.update(deltaTime, currentSpeed);
			this.nightMode.update(showNightMode);
			this.updateClouds(deltaTime, currentSpeed);

			if (updateObstacles) {
				this.updateObstacles(deltaTime, currentSpeed);
			}
		},

		/**
		 * Update the cloud positions.
		 * @param {number} deltaTime
		 * @param {number} currentSpeed
		 */
		updateClouds: function (deltaTime, speed) {
			var cloudSpeed = (this.cloudSpeed / 1000) * deltaTime * speed;
			var numClouds = this.clouds.length;

			if (numClouds) {
				for (var i = numClouds - 1; i >= 0; i--) {
					this.clouds[i].update(cloudSpeed);
				}

				var lastCloud = this.clouds[numClouds - 1];

				// Check for adding a new cloud.
				if (
					numClouds < this.config.MAX_CLOUDS &&
					this.dimensions.WIDTH - lastCloud.xPos > lastCloud.cloudGap &&
					this.cloudFrequency > Math.random()
				) {
					this.addCloud();
				}

				// Remove expired clouds.
				this.clouds = this.clouds.filter(function (obj) {
					return !obj.remove;
				});
			} else {
				this.addCloud();
			}
		},

		/**
		 * Update the obstacle positions.
		 * @param {number} deltaTime
		 * @param {number} currentSpeed
		 */
		updateObstacles: function (deltaTime, currentSpeed) {
			// Obstacles, move to Horizon layer.
			var updatedObstacles = this.obstacles.slice(0);

			for (var i = 0; i < this.obstacles.length; i++) {
				var obstacle = this.obstacles[i];
				obstacle.update(deltaTime, currentSpeed);

				// Clean up existing obstacles.
				if (obstacle.remove) {
					updatedObstacles.shift();
				}
			}
			this.obstacles = updatedObstacles;

			if (this.obstacles.length > 0) {
				var lastObstacle = this.obstacles[this.obstacles.length - 1];

				if (
					lastObstacle &&
					!lastObstacle.followingObstacleCreated &&
					lastObstacle.isVisible() &&
					lastObstacle.xPos + lastObstacle.width + lastObstacle.gap < this.dimensions.WIDTH
				) {
					this.addNewObstacle(currentSpeed);
					lastObstacle.followingObstacleCreated = true;
				}
			} else {
				// Create new obstacles.
				this.addNewObstacle(currentSpeed);
			}
		},

		removeFirstObstacle: function () {
			this.obstacles.shift();
		},

		/**
		 * Add a new obstacle.
		 * @param {number} currentSpeed
		 */
		addNewObstacle: function (currentSpeed) {
			var obstacleTypeIndex = getRandomNum(0, Obstacle.types.length - 1);
			var obstacleType = Obstacle.types[obstacleTypeIndex];

			// Check for multiples of the same type of obstacle.
			// Also check obstacle is available at current speed.
			if (this.duplicateObstacleCheck(obstacleType.type) || currentSpeed < obstacleType.minSpeed) {
				this.addNewObstacle(currentSpeed);
			} else {
				var obstacleSpritePos = this.spritePos[obstacleType.type];

				this.obstacles.push(
					new Obstacle(
						this.canvasCtx,
						obstacleType,
						obstacleSpritePos,
						this.dimensions,
						this.gapCoefficient,
						currentSpeed,
						obstacleType.width
					)
				);

				this.obstacleHistory.unshift(obstacleType.type);

				if (this.obstacleHistory.length > 1) {
					this.obstacleHistory.splice(Runner.config.MAX_OBSTACLE_DUPLICATION);
				}
			}
		},

		/**
		 * Returns whether the previous two obstacles are the same as the next one.
		 * Maximum duplication is set in config value MAX_OBSTACLE_DUPLICATION.
		 * @return {boolean}
		 */
		duplicateObstacleCheck: function (nextObstacleType) {
			var duplicateCount = 0;

			for (var i = 0; i < this.obstacleHistory.length; i++) {
				duplicateCount = this.obstacleHistory[i] == nextObstacleType ? duplicateCount + 1 : 0;
			}
			return duplicateCount >= Runner.config.MAX_OBSTACLE_DUPLICATION;
		},

		/**
		 * Reset the horizon layer.
		 * Remove existing obstacles and reposition the horizon line.
		 */
		reset: function () {
			this.obstacles = [];
			this.horizonLine.reset();
			this.nightMode.reset();
		},

		/**
		 * Update the canvas width and scaling.
		 * @param {number} width Canvas width.
		 * @param {number} height Canvas height.
		 */
		resize: function (width, height) {
			this.canvas.width = width;
			this.canvas.height = height;
		},

		/**
		 * Add a new cloud to the horizon.
		 */
		addCloud: function () {
			this.clouds.push(new Cloud(this.canvas, this.spritePos.CLOUD, this.dimensions.WIDTH));
		}
	};
})();

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	new Runner(".interstitial-wrapper");
}

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